Interpretive Summary: The protozoan parasite, Neospora caninum is a recently recognized cause of abortion in livestock. Its life cycle and sources of infection are unknown. Transplacental transmission is the only proven route of infection. In the present study authors conducted an extensive survey of neosporosis abortion in cattle in California. Scientists at the Beltsville Agricultural Research Center and the University of California found neosporosis abortion in 19 of the 26 dairy herds surveyed. Neospora caninum was identified in 113 of 266 (43.5%) of abortions. Control measures are not possible unless the source of infection is identified.

Technical Abstract:
A survey of abortions in California dairies involving a total of 19,708 cows was conducted to estimate the minimum rate of abortion attributable to Neospora infection. Fourteen herds with a history of Neospora abortions and 12 herds with no known Neospora abortions were selected for a prospective year-long survey. All available aborted fetuses were submitted to the California Veterinary Diagnostic Laboratory System to determine the cause of abortion. The major identified cause was Neospora infection, which was identified in 113 of 266 abortions. A majority (87%) of the abortions was submitted from herds with a previous history of Neospora abortion, and Neospora infection was identified in 43.5% of these abortions. Fewer aborted fetuses were submitted in those herds with no previous history of Neospora abortion; however, Neospora abortions were confirmed in 6 of the 12 herds and the infection was identified in 35.3% of abortions. The disease was geographically widespread throughout the state, occurring in 19 of the 26 herds surveyed. The available reproductive history of cows that had a Neospora abortion was evaluated and in 4 instances, cows were identified that had twice aborted a Neospora-infected fetus.